1. Field of the Invention
This invention relates to a radiation image read-out method in a radiation image recording and reproducing system wherein a stimulable phosphor sheet is used, and an apparatus for carrying out the method.
2. Description of the Prior Art
When certain kinds of phosphors are exposed to a radiation such as X-rays, .alpha.-rays, .beta.-rays, .gamma.-rays, cathode rays or ultraviolet rays, they store a part of the energy of the radiation. Then, when the phosphor which has been exposed to the radiation is exposed to stimulating rays such as visible light, light is emitted by the phosphor in proportion to the stored energy of the radiation. A phosphor exhibiting such properties is referred to as a stimulable phosphor.
As disclosed in U.S. Pat. No. 4,258,264 and Japanese Unexamined Patent Publication No. 56(1981)-11395, it has been proposed to use a stimulable phosphor in a radiation image recording and reproducing system. Specifically, a sheet provided with a layer of the stimulable phosphor (hereinafter referred to as a stimulable phosphor sheet) is first exposed to a radiation passing through an object to have a radiation stored therein, and is then scanned with stimulating rays such as a laser beam which cause it to emit light in the pattern of the stored image. The light emitted by the stimulable phosphor sheet upon stimulation thereof is photoelectrically detected and converted to an electric image signal, which is processed as desired to reproduce a visible image on a recording medium such as a photo sensitive material or on a display device such as a cathode ray tube (CRT).
FIG. 3 is a perspective view showing the conventional read-out apparatus used in the aforesaid radiation image recording and reproducing system for scanning a stimulable phosphor sheet carrying a radiation image stored therein by stimulating rays such as a laser beam which cause the sheet to emit light in proportion to the stored radiation energy, and photoelectrically detecting the emitted light.
In the apparatus of FIG. 3, stimulating rays 2 are emitted by a stimulating ray source 1, and the beam diameter of the stimulating rays 2 is strictly adjusted by a beam expander 3. The stimulating rays 2 are then deflected by a light deflector 4 formed of a galvanometer mirror or the like, and are made to impinge upon the stimulable phosphor sheet 10 by a plane reflection mirror 5. Between the light deflector 4 and the plane reflection mirror 5 is positioned an f.theta. lens 6 for maintaining the beam diameter of the stimulating rays 2 uniform during the constant-speed scanning of the stimulating rays 2 in the main scanning direction as indicated by the arrow A on the stimulable phosphor sheet 10. While the stimulating rays 2 impinge upon the stimulable phosphor 10, the sheet 10 is moved in a sub-scanning direction at a predetermined angle with respect to the main scanning direction, e.g. as indicated by the arrow B, and consequently, the whole area of the sheet 10 is exposed to and scanned by the stimulating rays 2. Upon exposure to the stimulating rays 2, the stimulable phosphor sheet 10 emits light in proportion to the radiation energy stored therein, and the light emitted enters a light guide member 8. The light guide member 8 has a linear light input face 8a positioned close to a scanning line 2a on the stimulable phosphor sheet 10, and a ring-shaped light output face 8b in close contact with the light receiving face of a photodetector 9, which may be a photomultiplier. The light guide member 8 and the photodetector 9 constitute a photoelectric read-out means 7. The light guide member 8 is fabricated of a transparent thermoplastic resin sheet such as an acrylic resin sheet so that the light entering from the light input face 8a can be transmitted to the light output face 8b by total reflection inside of the light guide member 8. The light emitted by the stimulable phosphor sheet 10 upon stimulation thereof is guided inside of the light guide member 8, emitted from the light output face 8b of the light guide member 8 and received by the photodetector 9. The light guide member 8 may be of a shape and a material as disclosed in U.S. Pat. No. 4,346,295.
The light detected by the photodetector 9 is converted thereby into an electric signal, and the output signal of the photodetector 9 is amplified, A/D converted and subjected to a signal processing. Thereafter, the electric signal is used for reproducing a visible image on a recording material such as a photographic film or on a display device such as a CRT.
However, when the time interval between radiation image recording on a stimulable phosphor sheet and image read-out therefrom is short in the aforesaid conventional radiation image read-out apparatus, not only the light emitted by the stimulable phosphor sheet in proportion to the stored radiation energy when the sheet is exposed to stimulating rays but also an instantaneous light emission after-glow (noise) are detected by the photodetector. Therefore, the radiation image is not detected accurately, and the contrast of the reproduced visible image becomes low. By "instantaneous light emission after-glow" is meant the after-glow of light instantaneously emitted by the stimulable phosphor when it is exposed to radiation for image recording. The instantaneous light emission after-glow continues to be emitted by the whole exposed surface of the stimulable phosphor sheet 10 for a fixed time after it is exposed to radiation.
Specifically, in the aforesaid conventional radiation image read-out apparatus, the light input face 8a of the light guide member 8 is positioned facing the scanning line 2a over the whole width of the stimulable phosphor sheet 10 in the main scanning direction, and all light entering the light guide member 8 from its light input face 8a is detected by the photodetector 9. Therefore, the radiation after-glow emitted by the whole width region of the stimulable phosphor sheet 10 standing face to face with the light input face 8a is detected by the photodetector 9, and the aforesaid problems are caused by the detected instantaneous light emission after-glow.
In the present invention, detection of the light emitted by the stimulable phosphor sheet in proportion to the stored radiation energy when it is scanned by stimulating rays is often referred to as read-out from the stimulable phosphor sheet.